System for measuring low current with contact making and breaking device

ABSTRACT

A contact making and breaking device improves the dielectric absorption property of capacitance between one signal wire brought out from a reed switch and an conductive casing. A low current measurement system using the contact making breaking device greatly shortens the measurement waiting time when low current is measured. The contact making and breaking device includes a reed switch with first and second signal wires brought out from either end, a conductive casing, and an insulating material formed between at least the first signal wire and the conductive casing. A tubular conductor is emplaced in the region in which the insulating material is formed and at least partially encloses the reed switch. The tubular conductor is connected to the first signal wire.

This is a divisional of application Ser. No. 08/626,790 filed on Apr. 2,1996, now U.S. Pat. No. 5,742,216.

FIELD OF THE INVENTION

This invention concerns a contact making and breaking device using areed switch in combination with a system for measuring low current. Morespecifically, it concerns a contact making and breaking device whichgreatly improves the dielectric absorption properties of capacitancebetween one signal wire brought out from the reed switch and an externalconductor. It further concerns a system which contains a current sourcethat uses the contact making and breaking device in a manner whichgreatly shortens (or reduces to zero) the measurement waiting time (thetime until the erroneous current becomes small enough not to affect themeasurement) when low current is measured.

BACKGROUND OF THE ART

FIG. 6 shows an example of a prior art system for measuring low current.In FIG. 6, a core wire of a reed relay 7 (signal line 71 brought outfrom reed switch 74) is connected to a signal output terminal (forceterminal 21) of a system which contains a current source and which hasan active guard. External conductive casing 72 of reed relay 7 isconnected to a guard terminal 22 of a system containing a current source2, through a switch 81, and to a ground terminal (guard terminal 12) ofa picoammeter 1 (pA in FIG. 6). The other core wire of reed relay 7(i.e., signal line 73) is connected to the signal input terminal (senseterminal 11) of picoammeter 1 and to the ground terminal (guard terminal12) through switches 82 and 83.

The drive coil of reed relay 7 is omitted in FIG. 6.

In this measurement system, when current flowing through force terminal2L from current source 2 is not being measured (i.e., duringnon-measurement period), the contact points of reed switch 74 andswitches 81 and 82 are opened, and switch 83 is closed. When the currentflowing through force terminal 21 from current source 2 is beingmeasured (i.e., during measurement period), an opposite configuration ispresent: i.e., reed switch 74 and switches 81 and 82 are closed, andswitch 83 is opened.

As mentioned above, signal wire 73 and conductive casing 72 are forcedto the same potential in both the current non-measurement and thecurrent measurement periods through the operation of reed switch 74 andswitches 81-83.

In the measurement system of FIG. 6, however, there are cases in whichthe voltage which appears at force terminal 21 differs during thecurrent non-measurement and current measurement periods. The operationin this case will be explained by referring to the equivalent circuit ofthe reed relay in FIG. 7. In FIG. 7, a1 and a2 are terminals which areconnected, respectively, to force terminal 21 and guard terminal 22.Terminals b1 and b2 are connected respectively to sense terminal 11 andguard terminal 12 of picoammeter 1. K1 is the contact point of the reedswitch, C1 the capacitance between signal lines 71 and 73, C22 is thecapacitance between signal wire 71 and external casing 72, and C21 isthe capacitance between signal wire 73 and external casing 72.

When a voltage appears at force terminal 21, that voltage charges signalline 71 and external casing 72 with a charge that is based on staticcapacitance C22.

When current is being measured, reed switch 74 and switches 81 and 82are closed, while switch 83 is opened, as mentioned above. If at thistime, there is only air in the capacitance space between signal wire 71and conductive casing 72, the charging current flows instantly intopicoammeter 1. However, in reed relay 7 shown in FIG. 6, reed switch 74is within conductive casing 72, so that insulating parts 75 and 76 areinterposed in the capacitance space. This produces the followingproblems.

Insulating parts 75 and 76 have dielectric absorption properties. Thedielectric absorption due to insulating part 75 prevents aninstantaneous discharge of the charge between signal wire 71 andconductive casing 72 (capacitance C22 in FIG. 7). Therefore, when theoutput current of force terminal 21 is measured by picoammeter 1, thecurrent measurement cannot be performed until a suitable time passesafter reed switch 74 is closed (i.e., there is a large measurementwaiting time).

For example, in the case of a conventional reed relay 7, if it isassumed that a voltage of 100 V appears on the signal wire 71(immediately before the current measurement), the value of the currentdue to the discharge of the charge between signal wire 71 and externalconductor 72 frequently requires several tens of seconds to fall to thefemto-ampere level.

This invention has the objective of providing a contact making andbreaking device which (I) greatly improves the dielectric absorptionproperties of capacitance between a signal wire emanating from a reedswitch and an external conductive casing, and (ii) greatly shortens (orreduces to zero) the measurement waiting time (the time until theerroneous current becomes small enough not to affect the measurement)when low current is measured.

The aforementioned problems are caused by:

(1) the fact that an insulating part is present which reduces thedielectric absorption property in the capacitance space (see C22 in FIG.7) between the signal wire and the conductive casing of the reed relaythat is connected to the system which contains a current source; and

(2) the fact that, during the non-measurement period, the current due tothe charge between the signal wire (connected to the system whichcontains a current source) and the external conductor (i.e., the chargebased on the existence of the aforementioned insulating part) flows intothe picoammeter as an error current when one tries to start a currentmeasurement (when the reed relay is closed).

SUMMARY OF THE INVENTION

The inventors, noting (1) and (2) above, have discovered that:

(1) the aforementioned problems can be solved by covering the spacebetween the signal wire and the external conductor with a tubularconductor, so that no insulating part exists in the capacitance spacebetween the signal wire and the external conductor; and

(2) since the aforementioned tubular conductor is placed in the externalconductive casing, some of insulating parts are present in thecapacitance space between the signal wire from the reed switch (thesignal wire on the side connected to the system which contains a currentsource) and the tubular conductor. A closed circuit is thus formed, sothat current due to the charge resulting from the insulating part doesnot flow into the picoammeter.

This invention comprises a contact making and breaking device which hasa reed switch with two signal wires brought out from either end and anexternal conductive casing, as well as an insulating part formed betweenat least one of the aforementioned signal wires and the externalconductive casing. A tubular conductor is placed in the region formed bythe insulating part between at least one of the signal wires and theexternal conductive casing. The tubular conductor is electricallyconnected to the other of the two signal wires.

In the contact making and breaking device of this invention, because ofthe presence of the tubular conductor, there is no insulating part, oran extremely small one, in the capacitance space formed between thesignal wire and the conductive casing. Therefore, capacitance betweenthe signal wire and the external conductor either produces no dielectricabsorption at all, or one which is negligibly small.

The contact making and breaking device of this invention also contains,besides a dry reed relay, called a "reed relay," a wetted reed relay,called a "mercury relay". As to the insulating material, it is desirableto use a material with an excellent dielectric absorption property, suchas polytetrafluoroethylene.

In this invention, insulating parts are present between the ends of theconductive casing and the two signal wires brought out from the ends ofthe reed switch. For example, an insulating part can be used which has acentral hole that abuts the tubular conductor (i.e., a "circularinsulating part"). The insulating part, the inner-diameter-end surfaceof which abuts the tubular conductor and the outer-diameter-end surfaceof which abuts the external conductive casing (i.e., the "ring-shapedinsulating part"), and the reed switch and the tubular conductor can beplaced inside the external conductive casing. The ends of the tubularconductor can also be made so that they terminate at the outer-endsurfaces of the insulating part.

It is preferable, for the ends of the tubular conductor to projectoutside the insulating part, between one end of the tubular conductor(on the side of one of the signal wires) and the conductive casing, sothat no insulating material is placed in the capacitance space betweenone of the signal wires and the conductive casing.

A tubular conductor can also be used which covers the whole of the reedswitch, and an insulating part or conductive part is formed at the endwhere the other signal wire is brought out. In this case, the othersignal wire is supported by the insulating part or conductive part.

If the other signal wire is supported by the insulating material, thesignal wires and the tubular conductor are electrically connected bywires. When the other signal wire is supported by the conductive part, ahole is made in the conductive part, through which the signal wire ispassed, and this signal wire is electrically connected to the tubularconductor, with the aforementioned conducting part between them.

In the contact making and breaking device of this invention, thespecific method by which the aforementioned signal wires are supportedby the insulating parts and the specific shape of the tubular conductorare not limited to those described above; this invention includes otherembodiments, which are not shown in the examples described below (FIGS.4A-C). Further, the positional relationships between the ends of theexternal conductor and the ends of the tubular conductor may be asdesired. Considering the operation of the guard with respect to thesignal wires, due to the external conductive casing, etc., it ispreferable for the ends of the external conductor to project outwardbeyond the ends of the tubular conductor.

In the system for measuring low current of this invention, the signalinput terminal of the ammeter is connected to the signal output terminalof the system that contains a current source, through the contact makingand breaking device which uses a reed switch. The measurement system ofthis invention assumes that the voltage which appears at the signaloutput terminal can be different when the current is being measured andwhen it is not being measured.

In the measurement system of this invention, the contact making andbreaking device is wired in such a way that one of the signal wires isconnected to the signal output terminal of the system that contains acurrent source and the other signal wire is connected to the signalinput terminal of the ammeter. Moreover, when the current is not beingmeasured, the connection is opened between the signal wire that isconnected to the system's signal output and the conductive casing. Whenthe current is being measured, the signal wire connected to the signaloutput terminal and the conductive casing is short-circuited through theammeter.

The current due to the charge between the signal wire from the reedswitch and the external conductor, while the current was not beingmeasured (corresponding to the charge C22 in FIG. 7), flows into thepicoammeter when the contact of the reed switch is closed. However,since there is no insulating part in the capacitance space formedbetween the signal wire and the external conductor, or at most a verysmall insulating part, the charge current instantly reaches zero.

The current due to the charge between the signal wire and the tubularconductor, when the current is not being measured (corresponding to thecharge C1 in FIG. 7), flows through the closed circuit formed when thecontact of the reed switch is closed Since there is an insulating partin the capacitance space between the signal wire and the tubularconductor, the current which flows through this closed circuit does notinstantly become zero. However, since this current does not flow intothe picoammeter, it has no effect on the current measurement.

In the system described above, there may also be an insulating partbetween the signal wire on the side on which the tubular conductor ofthe reed switch is connected (the other reed wire) to the conductivecasing, or in the capacitance space between the tubular conductor andthe external conductor (these correspond to C21 of FIG. 7). In thesystem, however, since the signal wire on the side to which the tubularconductor is connected is always at the same potential as the conductivecasing, no charge based on this capacitance is produced between thesignal wire on the side to which the tubular conductor is connected andthe conductive casing.

In the system for measuring low current of this invention, when the reedswitch is closed, the charge current flowing into the picoammeterinstantly becomes zero; therefore, a long waiting time is not needed forthe low current measurement.

BRIEF EXPLANATION OF FIGURES

FIG. 1A shows an example of the contact making and breaking device ofthis invention which uses a tubular conductor.

FIG. 1B shows an example of the contact making and breaking device ofthis invention which uses a tubular conductor with one closed end.

FIG. 1C shows an example of the contact making and breaking device ofthis invention which uses a tubular conductor the same length as in FIG.1(B), but with both ends open.

FIG. 2 is a diagram explaining the dielectric absorption of a contactmaking and breaking part (reed relay) of this invention.

FIG. 3A is a graph showing the relationship between time and thedischarge current due to the charge between the signal wire and theconductive casing in a conventional reed relay.

FIG. 3B is a graph showing the relationship between time and thedischarge current due to the charge between the signal wire and theconductive casing in a reed relay of this invention.

FIG. 4A is a diagram showing a form of the contact making and breakingdevice of this invention in which the end of the tubular conductor isbent into a flange shape.

FIG. 4B is a diagram showing a form of the contact making and breakingdevice of this invention in which the ring-shaped insulating part isdisplaced toward the center of the reed relay.

FIG. 4C is a diagram showing a form of the contact making and breakingdevice of this invention in which the insulating part is formed byfilling.

FIG. 5 is a diagram showing an example of the low current measurementsystem of this invention.

FIG. 6 is a diagram showing low current measurement system using aconventional reed relay.

FIG. 7 is a diagram showing an equivalent circuit of the reed relay inFIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-1C show an example of the contact making and breaking device ofthis invention (i.e., a dry reed relay). In reed relay 3A (FIG. 1A), acircular insulating part 351 with a central hole H1 is positioned insideof one end of a tubular conductive casing 32. A tubular conductor 37A isplaced so that it encloses circular insulating part 351. A ring-shapedinsulating part 352 is positioned between the outside of tubularconductor 37A and the inside of the conductive casing 32. A circularinsulating part 36 with a central hole H2 is positioned within the otherend of the conductive casing 32.

A reed switch 34 is placed between circular insulating parts 351 and 36to enable a signal wire 31, brought out from the reed switch 34, andanother signal wire 33, to pass through central holes H1 and H2. Tubularconductor 37A is connected to signal wire 33 by wire W. The ends E11 andE12 of tubular conductor 37A project to both sides of ring-shapedinsulating part 352. In this way, the device is made so that there areno insulating parts 352 or 351 in the capacitance space between signalwire 31 and conductive casing 32 (or if there is, it is extremelysmall). Therefore, there is almost no dielectric absorption betweensignal wire 31 and conductive casing 32.

The dielectric absorption of the contact making and breaking part (i.e.,the reed relay) will be explained below, with reference to FIG. 2.Furthermore, in the reed relay of FIG. 2 and the reed relay of FIG. 1A,the shapes of tubular conductors 37 differ slightly. Although this isnot shown in the FIG. 2, in tubular conductor 37 of FIG. 2, the partoutside of insulating part 351 is formed so that it projects, in thesame manner as in FIG. 1A (the projecting part is shown by the symbolE), and the part inside insulating part 351 extends toward the center ofthe reed relay.

In FIG. 2, when the outside piece of signal wire 31 is to the left ofpoint P (i.e., the piece of the wire indicated by ds'), the end E oftubular conductor 37 hides ring-shaped insulating part 352 from ds'.That is, no capacitance is present between ds' and conductive casing 32,or it is formed only by air. Therefore, the component of the capacitancedue to ring-shaped insulating part 352, between the part of the signalwire 31 to the left of point P and conductive casing 32, is zero, and nodielectric absorption is produced.

Moreover, when the outside piece of signal wire 31 extends to the rightof point P (i.e., the piece designated ds), ring-shaped insulating part352 can be seen from ds (the region in which it can be seen is shown bythe dot-and-dash line). That is, a capacitance is formed between ds andconductive casing 32 through the air and the dielectric (part ofinsulating part 352). Therefore, the dielectric absorption of thisdielectric remains a problem.

The component of the capacitance between ds and conductive casing 32that is due to insulating part 352 becomes small if ds is a certaindistance away from point P. Therefore, by choosing the length of theprojecting part of the tubular conductor 37 so that the capacitancecomponent due to insulating part 352 is made sufficiently small, thedielectric absorption of the capacitance between signal wire 31 and theconductive casing 32 can be reduced.

When, for example, the end E of the tubular conductor 37 does notproject outside the ring-shaped insulating part 352, ring-shapedinsulating part 352 is visible from the piece of the signal wire 31outside insulating part 352 (e.g., from the piece ds'). Nevertheless,the dielectric absorption produced between signal wire 31 and theconductive casing 32 is very small, compared to the dielectricabsorption of conventional reed relay 7 shown in FIG. 6. Therefore, ifthe demands placed on the dielectric absorption properties are not toostringent, one can make end E so that it does not project from the endsurface of ring-shaped insulating material 352.

In reed relay 3B of FIG. 1B, a tubular conductor which covers all ofreed switch 34 is used. This tubular conductor 37B has one closed end Bthat supports signal wire 33. Reed relay 3B of FIG. 1B has the samemake-up as reed relay 3A of FIG. 1A, except that it uses tubularconductor 37B instead of tubular conductor 37A and wire W. In FIG. 1B,end E2 of the tubular conductor 37B projects from the end of ring-shapedinsulating part 352, as in FIG. 1A. Moreover, closed end B is connectedelectrically to signal wire 33 inside insulating part 36 which supportsthe wire 33.

In reed relay 3C of FIG. 1C, a tubular conductor 37C is used, which hasabout the same length as tubular conductor 37B and is open at both ends.A circular insulating part 353 which has a central hole H3, is placedinside tubular conductor 37C on the end which is on the side of signalwire 33, and signal wire 33 passes through this central hole H3. Tubularconductor 37C is connected to signal wire 33 by a wire W.

In FIG. 1C, the left end part of tubular conductor 37C can also be madeso that it penetrates through insulating part 36. In this case, theshape of the end of the reed relay on the side of insulating part 36 canbe made with the same structure as the ends on the sides of insulatingparts 351 or 352 in FIGS. 1A-C, or it can be made with the samestructure as the ends of the sides of the insulating parts 351 and 352in the reed relay of FIG. 4B, as described below. In either case,tubular conductor 37C is connected to signal wire 33 with a wire Woutside insulating part 36.

FIGS. 3A and 3B are graphs showing a comparison of the properties ofconventional reed relay 7, shown in FIG. 6, and reed relay 3, shown inFIG. 1B. FIGS. 3A and 3B show the relationship between the chargecurrent flowing between the terminals and elapsed time between the timewhen the contact points of the respective reed relays are first open; avoltage of 100 V is applied for 60 seconds between one signal wire andthe conductive casing; and then the reed relay contact point is closed.

As can be seen from FIGS. 3A and 3B, in conventional reed relay 7,approximately 7.5 seconds are required for the discharge current tobecome 10 fA, while in reed relay 3, shown in FIG. 1B, the dischargecurrent becomes zero almost instantly. This affect is also shown by thereed relays of FIGS. 1A and 1C and FIGS. 4A-C, described below.

The contact making and breaking device of this invention is not limitedto the examples described above; one can employ various methods ofsupporting the signal wires of the reed switch by insulating parts, andthe tubular conductor employed may have various shapes.

FIGS. 4A and 4B are enlarged diagrams of the end of tubular conductor 37of the reed relay. In reed relay 3 of FIG. 4A, the projecting part ofthe open end of tubular conductor 37 is bent outward, forming a flangedshape designated by F. In FIG. 4B, only ring-shaped insulating part 352is placed towards the center of the reed relay 3.

In FIG. 4A, flanged part F, and in FIG. 4B, the position of ring-shapedinsulating part 352, act in such a way that no insulating material ispresent in the capacitance space between wire 31 and conductive casing32. Therefore, even if the projecting length of the end of tubularconductor 37 is short (or it does not project at all), the same effectcan be obtained as when the projecting length is made long.

FIG. 4C shows a reed relay which is formed by causing insulatingmaterial to fill the space inside the conductive casing In FIG. 4C,insulating material 38 fills this space instead of insulating parts 351,352, and 36 in the reed relay shown in FIG. 1A. Tubular conductor 37A,which is configured longer than tubular conductor of FIG. 1(A), issupported inside conductive casing 32 by insulating material 38. In FIG.4C, a conductive film 39 is formed on the outer surface of the reedswitch, and does not touch signal wire 31 but is shorted to signal wire33. Conductive film 39 also acts in such a way that there is no effectsof insulating parts in the capacitance space between the wire inside theglass tube of the reed switch and conductive casing 32.

The various reed relays described above can be applied to multi-pathrelays or multi-path break relays, wherein the same structures areconnected in parallel. Moreover, they can also be applied to mixedmake/break relays, transfer relays, etc.

FIG. 5 shows low current measurement system of this invention, using theaforementioned reed relay. Here, for convenience, the explanation willuse the reed relay of FIG. 1B, but one can also use the reed relaysshown by FIGS. 1A, 1C and FIGS. 4A-C. This low current measurementsystem has the same structure as the low current measurement systemalready explained in FIG. 6, except that the reed relay is different,and the system that contains a current source 2 has an active guard.

In FIG. 5, the operations of reed relay 3 and switches 81-83 when acurrent is being measured and when it is not being measured are also thesame as those in the low current measuring system of FIG. 6. That is,when a current is not being measured, the contact points of the reedswitch 34 and the switches 81 and 82 are opened, and that of the switch83 is closed. When a current is being measured, the contact points ofthe reed switch 34 and the switches 81 and 82 are closed, and that ofthe switch 83 is opened.

In the low current measurement system of FIG. 5, a tubular conductor 37Bis placed in the insulating part between signal wire 31 and conductivecasing 32. Moreover, since the open end of the tubular conductor 37Bprojects beyond insulating parts 351 and 352, as described above, thereis almost no effects of insulating parts in the capacitance spacebetween conductive casing 32 and signal wire 31. Therefore, even ifthere is a voltage of a certain value at force terminal 21 immediatelybefore the current measurement is started, the current due to the chargebetween the conductive casing 32 and signal wire 31 flows instantly intopicoammeter 1 when the contact point of the reed switch 34 is closed.Moreover, the current due to the charge between tubular conductor 37Band signal wire 31 flows through the contact point of the reed switch34.

This current does not become zero instantaneously, due to the dielectricabsorption of the circular insulting material 351 between tubularconductor 37B and signal wire 31, but since this current does not flowinto picoammeter 1, it does not affect the current measurement.

The low current measurement system of this invention can be employed invarious systems that do not have active guards (e.g., devices formeasuring static capacities, current source devices, pulse generators,etc.), as well as in systems that contain current sources and that haveactive guards (e.g., systems containing switch matrices).

The following effects can be obtained by means of the inventiondescribed above.

(1) Since the insulating material contained in the capacitance spacebetween one signal wire and the conductive casing of the reed switch isreduced, a contact making and breaking device can be provided in whichthe effect of dielectric absorption is reduced.

(2) By devising a method of connecting the signal wire and the guardbefore and after the measurement, using a contact making and breakingdevice with reduced dielectric absorption, a low current measurementsystem is provided which has an extremely short measurement waiting timewhen low current is measured.

While the invention is described in terms of a preferred embodiment,variations may be made without departing from the spirit and scope ofthe invention as claimed below.

We claim:
 1. A system for measuring low current from a current source,comprising:a signal output terminal connected to said current source; anammeter having a signal input terminal connected to the signal outputterminal through a contact making and breaking device including a reedswitch, said reed switch including two terminals, a first signal wireconnected to a first of said terminals and said signal output terminaland a second signal wire connected to a second of said terminals andsaid ammeter; an external conductive casing positioned about said reedswitch; insulating means formed between said first signal wire and theexternal conductive casing; a tubular conductor positioned in theinsulating means and between said first signal wire and the externalconductive casing for decoupling said first signal wire from saidexternal conductive casing; means for electrically connecting thetubular conductor to the second signal wire; conductive means connectedbetween said second signal wire and said external conductive casing;first switch means being operative when a current is not being measuredby said ammeter, for opening said conductive means; and second switchmeans being operative when a current is being measured by said ammeter,for connecting said second signal wire to said external conductivecasing through the ammeter.
 2. A system for measuring low current inaccordance with claim 1, wherein the tubular conductor projects from theinsulating means.
 3. A system for measuring low current in accordancewith claim 1, wherein the reed switch is entirely contained within thetubular conductor.
 4. A system for measuring low current in accordancewith claim 1, wherein the electrically connecting means comprises awire.
 5. A system for measuring low current in accordance with claim 1,wherein the electrically connecting means comprises a conductive closingmeans closing an end of the tubular conductor.
 6. A system for measuringlow current in accordance with claim 2, wherein the open end of theprojected tubular conductor is bent outward.